Efficient conversion of exhaust gas emissions in a gasoline engine includes maintaining the catalyst feedgas air-fuel ratio at a narrow window around stoichiometry. However, during actual engine operation, slight excursions away from stoichiometry may occur. To increase the operating window and thus improve emissions performance, catalysts often include ceria to provide a buffer for oxygen storage. To maintain optimal catalyst performance, stored oxygen may be maintained at a desired set point, calibrated based on engine load and temperature, via feedback control of engine air-fuel ratio.
However, the inventors herein have recognized an issue with the above approach. Determining the level of stored oxygen in a catalyst typically involves utilization of a physics-based catalyst model that includes a plurality of partial differential equations in one or more dimensions. Such a model may be difficult to implement and may require more processing power than typically available in an engine controller.
Thus in one example, the above issue may be at least partly addressed by a method for an engine exhaust system. In one embodiment, the method comprises adjusting a fuel injection amount based on a fractional oxidation state of a catalyst, the fractional oxidation state based on reaction rates of a plurality of exhaust gas species throughout a catalyst longitudinal axis and a set of axially-averaged mass balance and energy balance equations for a fluid phase and a washcoat of the catalyst, and based on feedback from a downstream air-fuel ratio sensor.
In another example, an engine exhaust method, comprises adjusting a fuel injection amount based on: a fractional oxidation state (FOS) of a catalyst relative to an FOS set-point, the FOS based on reaction rates of a plurality of exhaust gas species throughout a catalyst longitudinal axis and a set of axially-averaged mass balance and energy balance equations, and separate feedback from a downstream HEGO sensor relative to a HEGO set-point, the FOS and HEGO set-points tied together.
The present disclosure may offer several advantages. For example, processing resources devoted to the catalyst model may be reduced. Further, emissions control may be improved by maintaining the catalyst at a desired fractional oxidation state. In addition, the evolution of exhaust species, such as HC, NOx and CO, or aggregate oxidants and reductants, may be monitored, and if breakthrough is predicted, an operator of the vehicle may be notified and/or additional engine control operations may be undertaken to control the production of the exhaust species. Another advantage of the present approach is that it offers a non-intrusive catalyst monitor for control and diagnostics, which is less dependent on sensor location and hence will be equally applicable to both partial and full volume catalyst systems. Finally, by tying together the two set-points in this way, controller robustness can be improved while limiting complexity and calibration efforts.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.